Kidney stone is an exceedingly common disease. The lifetime risk of developing a kidney stone is about one in eight in American men and one in twenty in American women. An overwhelming majority of the kidney stones are recurrent, adding significantly to the pain, suffering and medical expenses. Kidney stone formation or nephrolithiasis is believed to be a multistage process, but precisely how each step takes place in a physiological environment remains poorly understood to date. In the past, attempts to understand the pathogenesis of kidney stone depended heavily on in vitro experimentation and human specimens harboring full-fledged stones. While extremely necessary, these approaches alone do not allow a comprehensive analysis of the full-spectrum of the lithogenic process from the beginning to the end. Over the last 10 years, our laboratory has been developing genetically engineered mouse models to characterize the sequential steps of nephrolithiasis. In particular, we inactivated the gene encoding Tamm-Horsfall protein (THP; also named uromodulin), a major urinary protein made by kidney's thick ascending limb of loop of Henle. We found that these knockout (KO) mice are prone to developing intra-renal calcinosis that bears strong resemblance in many respects to human patients bearing idiopathic calcium oxalate stones. The main goal of this renewal proposal is to significantly expand and deepen our understanding of the molecular and cellular mechanisms whereby defects of THP lead to intra-renal calcification. Specifically, we will investigate how interstitial calcification in THP KO mice originate and evolve in a spatial and temporal manner by performing ultra- structural and chemical and protein composition analyses. We will examine how renal epithelial cells uptake the intratubular crystals and whether and how this leads to cytotoxicity. We will determine whether formation of bona-fide kidney stones in THP KO mice relies on heightened urinary supersaturation of calcium phosphate or calcium oxalate by generating compound, genetically engineered mice that naturally develop these conditions. Together, these interconnected and mutually enforcing studies should offer novel insights into how kidney stones develop and how they can be better prevented.